This invention relates to an electrolytic process for the production of concentrated sodium hydroxide from a concentrated sodium chloride solution in an electrolytic cell in which the anolyte and the catholyte are separated from each other by a membrane.
It is known to make aqueous sodium hydroxide solutions (often referred to in the industry as caustic) by the electrolysis of brine (i.e., aqueous sodium chloride) in an electrolytic cell having an anode and a cathode, in which the anolyte and catholyte are separated by a hydraulically impermeable cationic membrane. Depending on the electrolysis conditions, the resulting sodium hydroxide solutions may have concentrations of about 20-55 weight percent NaOH, and the current efficiencies may vary from as low as about 60% to as high as about 97%, lower concentration sodium hydroxide solutions being usually obtained at higher current efficiencies. Since commercial caustic normally has a 50% NaOH concentration, more dilute solutions naturally must be further concentrated to that level, using either steam or some other source of energy. Caustic of at least about 45 weight % NaOH made in the electrolysis process can be directly concentrated to 50 % caustic using its own sensible heat. Accordingly, it would be desirable to be able to make aqueous sodium hydroxide solutions having high NaOH concentrations by brine electrolysis at a high current efficiency, especially solutions of at least 45% NaOH concentration, which can be concentrated to 50% NaOH commercial product without further expenditure of energy.
In addition to the basic requirement that concentrated caustic be made at a high current efficiency, it is important for the practical operation of such a process that this high current efficiency be maintained for an extended period. Thus, although various literature reports and patents suggest that high concentrations of caustic have been obtained at a fairly high current efficiency, very scant information is provided on the stability of the process over extended periods, and there is reason to believe that many of those processes would not maintain their efficiencies for months of continuous operation.
While considerable effort has been expended in this area and still is continuing worldwide, representative art includes th following:
U.S. Pat. No. 4,025,405 (Dotson et al., Diamond Shamrock), which describes brine electrolysis with a membrane having all sulfonyl groups, wherein it is reported that caustic of up to 55% NaOH was made without water addition in steady state operation. The examples, however, show that the current efficiency never exceeded 89.5% at a caustic concentration of 42% NaOH.
Japanese Laid Open Patent Application 55/158283 (Asahi Glass), which discloses a process using an all carboxyl group membrane with an equivalent weight of 690. An example describes the production of 55% caustic at a current efficiency of 88%.
Japanese Laid Open Patent Application 60/243292 (Asahi Glass) reports the production of 45% caustic at a 90-91.5% current efficiency using sulfonyl membranes. This application explicitly notes the instability of carboxylic acid groups when exposed long-term at high temperatures to caustic soda of a high concentration.
U.S. Pat. No. 4,202,743 (Oda et al., Asahi Glass) describes the production of 40% caustic with a carboxyl membrane having an equivalent weight of 667 at an initial current efficiency of 97% (after 10 days on line), which decreased to 93% at 360 days on line.
U.S. Pat. No. 4,135,996 (Buoy et al., Rhone-Poulenc) describes brine electrolysis with a nonporous perfluorosulfonate membrane, wherein a microporous layer such as asbestos or polytetrafluoroethylene was placed next to the cathode. Caustic of 44% NaOH concentration was made at a current efficiency of 60-61%.
U.S. Pat. No. 4,212,713 (Suhara, Asahi Glass) teaches that more dilute caustic can be made with a carboxyl membrane of higher equivalent weight (i.e., lower ion exchange capacity). However, it also notes that carboxylic acid groups of the membrane are remarkably unstable and are gradually decomposed in an alkali metal hydroxide solution at a concentration higher than 40 wt. % at a temperature higher than 80.degree. C.
Finally, a paper entitled "Operational Technology with "Flemion" For Chlor-Alkali Electrolysis" by K. Sato of Asahi Glass Co. at the May, 1986, meeting of the Electrochemical Society in Boston shows the inability of the state of the art membranes to make concentrated caustic. A specific membrane described in that paper could not tolerate operation at 45% caustic concentration for even a few hours, while operation at a 38% caustic concentration for two months at 70.degree. C. caused irreparable damage to the membrane.
As can be seen from the above discussion of some representative patent and technical literature, there still is no concensus of opinion as to satisfactory process conditions for producing high concentration caustic by electrolysis of brine in an electrolytic cell in which the anolyte and the catholyte are separated by a water-impermeable membrane.